The present invention relates generally to electronic communication systems, and in particular, to systems and methods for transmitting and receiving information from such systems over a computer network.
With the increasing popularity of the Internet and other content-heavy electronic communication systems, there has been a substantial need for reliable and affordable high bandwidth mediums for facilitating data transmissions between service providers and their customers. In relation to the requirement that such mediums be affordable to consumers, it was determined that the most cost-effective manner for providing service to customers was by using infrastructure already present in most locations. Accordingly, over recent years, the two such mediums most widely meeting these requirements include the cable television (CATV) and the conventional copper wire telephone systems (plain old telephone system or POTS).
In order to transmit and receive data over these mediums, most modern telecommunications systems utilize some type of modem to package, transmit and receive data over a physical medium such as conventional copper telephone lines, fiber optic networks, wireless networks, etc. Generally speaking, a modem is a generic term for any of a variety of modulator/demodulator (hence the term “modem”) devices, which, upon transmission, essentially format digital data signals into signals compatible with the type of network being utilized. In the case of conventional telephone modems, a modem operates to modulate a data signal generated by a computer into an analog format compatible with the PSTN (public switched telephone network). Such modulation may be accomplished in any of a variety of manners, dependent only upon the network protocol as well as the bandwidth capability of the physical medium being used. Examples of modulation techniques may include discrete multi-tone (DMT) modulation, frequency shift keying (FSK), phase shift keying (PSK), differential phase shift keying (DPSK), quadrature amplitude modulation (QAM), carrierless amplitude and phase (CAP) modulation. Essentially, these techniques conduct a bitwise conversion of the digital signal into a corresponding analog signal having a frequency related to the original digital value. In a similar manner to the transmission modulation techniques, modems also operate to receive and demodulate signals back into digital formats readable by a receiving terminal.
As the need for higher speed networks has increased, technology has developed which enables conventional networks to surpass the conventional bandwidth limitations of the PSTN network (i.e., a single 3000 Hz signal transmitted between a user and the phone company's nearest central office (CO)). One such technology generating significant interest is Digital Subscriber Line technology or DSL. Unlike a conventional modem, a DSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and the nearest CO. This dedicated copper wire can carry far more data than the 3,000 hertz signal needed for your phone's voice channel. By equipping both the user and the CO with DSL modems, the section of copper wire between the two can act as a purely digital high-speed transmission channel having a capacity on the order of 16 Mbps (million bits per second). In essence, a DSL modem operates to utilize the otherwise unused portion of the available bandwidth in the copper lines, i.e., the bandwidth between 24,000 and 2,200,000 Hz.
All types of DSL essentially operate by formatting signals using various Time Domain Equalization (TDE) techniques to send packets over the conventional copper wire at high data rates. In some circumstances, a substandard of conventional DSL known as Asymmetric Digital Subscriber Line+ (ADSL+) is considered advantageous for its ability to provide very high data rates in the downstream (i.e., from service provider to the user) direction by sacrificing speed in the upstream direction. Consequently, end user costs are minimized by providing higher speeds in the most commonly used direction. Further, ADSL+ provides a system that applies signals over a single twisted-wire pair that simultaneously supports (POTS) service as well as high-speed duplex (simultaneous two-way) digital data services.
FIG. 1 is a simplified block diagram of one embodiment of a typical architecture 100 for a G.992.2 splitterless ADSL+ system. A user's computer 102 is coupled to an ADSL+ modem 104 and to a conventional telephone line 106. Similarly, a conventional telephone 108 is also connected to line 106 for communication over voice band frequencies. Upon exiting the customer premises, line 106 relays information on the line to a telecom provider's central office 110. The central office 110 includes a DSL modem and necessary equipment to establish a link to, for example, the Internet or other electronic communication network.
As briefly described above, all DSL system operate in essentially the following manner. Initial digital data to be transmitted over the network is formed into a plurality of multiplexed data frames and encoded using special digital modems into analog signals which may be transmitted over conventional copper wires at data rates significantly higher than voice band traffic. The length and characteristics of wire run from a customer's remote transceiver to a central office transceiver may vary greatly from user to user and, consequently, the possible data rates for each user also vary. In addition, the physical channel (i.e., the wires themselves) over which the system communicates also vary over time due to, for example, temperature and humidity changes, fluctuating cross-talk interference sources, etc. Consequently, analog DSL signals exists in a noisy, time varying environment. Accordingly, all DSL systems use sophisticated training techniques as well as various forms of performance monitoring methodologies to ameliorate these factors.
In addition to noise conditions effecting the accurate reception of ADSL+ signals, limitations have also been placed on the effect ADSL+ transmissions may have upon outside signals occupying frequency bandwidths which may overlap those of the ADSL signals. In particular, due to potential interference with amateur radio and AM radio transmissions in the 1.8 to 2 MHz band, ADSL+ transmissions may not exhibit a power spectral density (PSD) above −80 dBm/Hz within this band. This interference is commonly referred to as RFI or radio frequency interference. Unfortunately, in maximizing transmission speeds using ADSL+ technology, it has been found that signals in at least one portion of the frequency range may exceed this limitation, resulting in unacceptable interference.
Therefore, there is a need for a system and method for optimally transmitting signals in an ADSL+ network wherein the PSD for the signals is maintained below −80 dBm/Hz.